The following invention relates to a forced diffusion process for making semiconductor material and in particular relates to a process for doping diamond material with impurities to make N-type diamond semiconductors which may then be used in electronics (e.g., high voltage switches, integrated circuits) optics, and other applications.
All semiconductors such as silicon, which are capable of being doped with impurities to create IC chips of various circuit configurations, are subject to limitations in high temperature, chemically corrosive, and high radiation environments. Diamond is one of the best insulators known, but heretofore it has been very difficult to produce diamond semiconductor material. Some success has been attained at creating P-type diamond semiconductors, but the production of N-type diamond has been largely unsuccessful. P-type diamond material has been made using in situ doping and ion implantation. N-type diamond has not been made by any process except for the process of the invention to be described herein. Furthermore, in situ doping and ion implantation introduce undesirable lattice defects in the diamond when P-type impurities are used.
Forced diffusion is a method of doping semiconductor material with impurities that has been used in the past with semiconductor material such as silicon. In a forced diffusion process, semiconductor material with impurity material placed on it is first heated Dopant material in the lattice is then activated by the heat (i.e., it becomes ionized). An electric field created by a high voltage placed across the silicon material drives the charged dopant atoms into the semiconductor lattice structure.
Heating alone may be sufficient to ionize only shallow level impurity dopants, but the major problem with forced diffusion is to diffuse enough impurity atoms in the lattice structure to contribute to conduction. The number of atoms is a function of temperature and activation energy according to the Boltzmann distribution. The higher the temperature, the more that impurity ions are created which will diffuse into the lattice structure. The problem is that with diamond the lattice structure begins to break down into amorphous carbon or graphite when temperature levels high enough to cause ionization are reached due to metastability of the diamond phase.
Forced diffusion has been achieved for impurity in diamond exhibiting P-type conduction and examples are noted in a paper entitled "Diffusion of Impurities Under Bias in CVD Diamond Films," Popovici et al Materials Research Society, Apr. 1994. This paper notes that diamond doping is difficult because of lattice rigidity and the small covalent radius of carbon. This means that only boron and nitrogen are potentially available since only these dopants have covalent radii small enough to enter the diamond lattice.
Diamond semiconductors of N-type material require high enough concentrations of dopant atoms with shallow energy levels in order to provide the requisite degree of conductivity. Thus far, diffusion doping processes have not been capable of achieving this result. Ion implantation and in situ doping have the problems associated with them that are noted above. What is needed, therefore, is a doping process for diamond material that can create N-type semiconductor material.